## ADDED Requirements

### Requirement: Backup JSON format definition
The system SHALL use a versioned JSON format for playlist backup files. The JSON structure SHALL contain the following top-level fields:
- `version` (number): Format version, currently `1`
- `exportTime` (string): ISO 8601 timestamp of export
- `appVersion` (string): Application version at export time
- `playlists` (array): Array of playlist objects

Each playlist object SHALL contain: `id`, `name`, `coverPath`, `description`, `createTime`, `updateTime`, `songCount`, `sortOrder`, and a `songs` array. Each song entry SHALL contain: `songFilePath`, `addTime`, `sortOrder`.

#### Scenario: Valid backup file structure
- **WHEN** a backup file is generated
- **THEN** the file SHALL be valid JSON containing `version`, `exportTime`, `appVersion`, and `playlists` fields
- **THEN** each playlist entry SHALL include its complete metadata and associated songs array

#### Scenario: Empty playlist database
- **WHEN** the user has no playlists and triggers an export
- **THEN** the system SHALL generate a valid JSON file with an empty `playlists` array

### Requirement: Export playlists to JSON file
The system SHALL allow users to export all playlists and their associated songs to a single JSON file saved to local storage. The system SHALL use the system `DocumentViewPicker` to let the user choose the save location. The exported file name SHALL follow the pattern `playlist_backup_YYYYMMDD_HHmmss.json`.

#### Scenario: Successful export to local storage
- **WHEN** the user triggers playlist export from the backup management UI
- **THEN** the system SHALL read all playlists and their songs from `PlaylistTable`
- **THEN** the system SHALL serialize the data to the defined JSON format
- **THEN** the system SHALL open a `DocumentViewPicker` for the user to choose a save location
- **THEN** the system SHALL write the JSON file to the chosen location
- **THEN** the system SHALL display a success toast with the file name

#### Scenario: Export with large dataset
- **WHEN** the database contains more than 100 playlists
- **THEN** the system SHALL query playlists in batches to avoid excessive memory usage
- **THEN** the export SHALL complete successfully without out-of-memory errors

#### Scenario: Export failure
- **WHEN** the user cancels the file picker or a write error occurs
- **THEN** the system SHALL display an error toast describing the failure
- **THEN** no partial file SHALL be left on disk

### Requirement: Import playlists from JSON file
The system SHALL allow users to import playlists from a previously exported JSON backup file. The system SHALL use `DocumentViewPicker` to let the user select the backup file.

#### Scenario: Successful import with no conflicts
- **WHEN** the user selects a valid backup JSON file
- **THEN** the system SHALL parse and validate the JSON structure and version
- **THEN** the system SHALL create all playlists and song associations from the backup
- **THEN** the system SHALL display a success toast with the count of imported playlists

#### Scenario: Import with version validation
- **WHEN** the user selects a backup file with an unsupported `version` number
- **THEN** the system SHALL display an error message stating the file version is not compatible
- **THEN** no data SHALL be imported

#### Scenario: Import with malformed JSON
- **WHEN** the user selects a file that is not valid JSON or missing required fields
- **THEN** the system SHALL display an error message indicating the file format is invalid
- **THEN** no data SHALL be imported

### Requirement: Import conflict resolution
When importing playlists, the system SHALL detect name conflicts with existing playlists and present the user with resolution options: "overwrite", "skip", or "rename" (append a numeric suffix).

#### Scenario: Conflicting playlist name — user chooses overwrite
- **WHEN** the imported backup contains a playlist with the same name as an existing local playlist
- **AND** the user selects "overwrite"
- **THEN** the system SHALL delete the existing playlist and its song associations
- **THEN** the system SHALL create the playlist from the backup data

#### Scenario: Conflicting playlist name — user chooses skip
- **WHEN** the imported backup contains a playlist with the same name as an existing local playlist
- **AND** the user selects "skip"
- **THEN** the system SHALL not modify the existing playlist
- **THEN** the system SHALL proceed to import the remaining playlists

#### Scenario: Conflicting playlist name — user chooses rename
- **WHEN** the imported backup contains a playlist with the same name as an existing local playlist
- **AND** the user selects "rename"
- **THEN** the system SHALL create the playlist with a suffixed name (e.g., "My Playlist (2)")
- **THEN** the system SHALL import all associated songs under the renamed playlist

#### Scenario: Multiple conflicts — apply to all
- **WHEN** multiple playlists in the backup conflict with existing playlists
- **THEN** the conflict resolution dialog SHALL provide an "apply to all" checkbox
- **THEN** choosing "apply to all" SHALL apply the selected resolution strategy to all remaining conflicts

### Requirement: Backup management UI entry
The system SHALL provide a "Backup & Restore" entry in the Settings page that navigates to a dedicated backup management interface.

#### Scenario: Navigate to backup management
- **WHEN** the user taps "Backup & Restore" in Settings
- **THEN** the system SHALL navigate to the backup management page
- **THEN** the page SHALL display options for "Export to File", "Import from File", "Backup to WebDAV", and "Restore from WebDAV"